Method for improving residual stress in tubular body

Abstract

An object is to provide a method and an apparatus for improving a residual stress in a tubular body, which are enabled to improve the residual stress reliably by clearly defining controlling rage for treatment conditions without depending on an installation state and configuration of the tubular body. When a cylindrical tubular body ( 2 ) is improved in its residual stress by locally irradiating an outer-circumferential surface of a welded portion (C) of the tubular body ( 2 ) with laser beams ( 5 a ) and by moving an irradiation area (s) in an circumferential direction, a plurality thermocouples ( 9 ) are installed on the tubular body ( 2 ) to be improved, a temperature history of the outer surface of the tubular body ( 2 ) by the irradiation of the laser beam ( 5 a ) is managed by measuring the temperature history itself.

The invention claimed is: 
     
       1. A tubular-body residual-stress improving method for improving residual stress of a cylindrical tubular body by locally irradiating an outer-circumferential surface of a welded portion of the tubular body with laser beams and by moving an irradiation area in a circumferential direction, wherein,
 a plurality of thermometers are installed on a tubular body to be improved; 
 a temperature history of an outer surface of the tubular body is measured by the plurality of thermometers while the outer surface is irradiated with the laser beams; 
 a highest achieving temperature, a temperature-rising time required for reaching the highest achieving temperature, and a heating length in an axial direction in which the highest achieving temperature stays in a predetermined temperature range are obtained; and 
 a heating width in the circumferential direction is controlled on the basis of a product of the temperature-rising time and a speed of moving the laser beams in the circumferential direction. 
 
     
     
       2. A tubular-body residual-stress improving method for improving residual stress of a cylindrical tubular body by locally irradiating an outer-circumferential surface of a welded portion of the tubular body with laser beams and by moving an irradiation area in a circumferential direction, wherein,
 preliminarily, a plurality of thermometers are installed on a different tubular body having a same condition as that of a tubular body to be improved, 
 a temperature history of an outer surface of the different tubular body is measured by the plurality of thermometers while the outer surface is irradiated with the laser beams; a highest achieving temperature, a temperature-rising time required for reaching the highest achieving temperature, and a heating length in an axial direction in which the highest achieving temperature stays in a predetermined temperature range are obtained; then a heating width in the circumferential direction is obtained on the basis of a product of the temperature-rising time and a speed of moving the laser beams in the circumferential direction, and 
 subsequently, when the tubular body to be improved is irradiated with the laser beams, the highest achieving temperature, the temperature-rising time, the heating length in the axial direction, and the heating width in the circumferential direction are controlled as treatment conditions. 
 
     
     
       3. The tubular-body residual-stress improving method according to  claim 1  or  2 , wherein
 a dimensionless time F=(τ×k)/h 2  is obtained, where τ is the temperature-rising time, k is a thermal diffusivity of the tubular body, and h is a wall thickness of the tubular body, and 
 the dimensionless time F is controlled as the temperature rising time. 
 
     
     
       4. The tubular-body residual-stress improving method according to  claim 3 , wherein the dimensionless time F is controlled with an upper limit and a lower limit thereof. 
     
     
       5. The tubular-body residual-stress improving method according to  claim 1  or  2 , wherein
 a dimensionless distance in the circumferential direction G=W/√(rh) is obtained, where W is the heating width in the circumferential direction, r is an average radius of the tubular body, and h is the wall thickness of the tubular body, and 
 the dimensionless distance G is controlled as the heating width in the circumferential direction. 
 
     
     
       6. The tubular-body residual-stress improving method according to  claim 5 , wherein a lower limit of the dimensionless distance G is controlled. 
     
     
       7. The tubular-body residual-stress improving method according to  claim 1  or  2 , wherein
 a dimensionless distance in the axial direction J=L/√(rh) is obtained, where L is the heating length in the axial direction, r is the average radius of the tubular body, and h is the wall thickness of the tubular body, and 
 the dimensionless distance J is controlled as the heating length in the axial direction. 
 
     
     
       8. The tubular-body residual-stress improving method according to  claim 7 , wherein the dimensionless distance J is controlled to be not less than 3.0. 
     
     
       9. The tubular-body residual-stress improving method according to  claim 1  or  2 , wherein the highest achieving temperature is controlled:
 to be not lower than 550° C. and lower than 650° C. in a case where the tubular body is made of an austenitic stainless steel; 
 to be not lower than 550° C. and lower than 650° C. in a case where the tubular body is made of a nickel-chromium iron alloy; and 
 to be not lower than 500° C. and lower than 595° C. in a case where the tubular body is made of any one of a low-alloy steel and a carbon steel. 
 
     
     
       10. The tubular-body residual-stress improving method according to  claim 1  or  2 , wherein a product (v×h) is controlled to be not less than 70 mm 2 /s, where v is the moving speed of the laser beams and h is the wall thickness of the tubular body. 
     
     
       11. The tubular-body residual-stress improving method according to  claim 3 , wherein a lower limit of the dimensionless time F is controlled in a case where an inner surface of the tubular body is cooled by water. 
     
     
       12. The tubular-body residual-stress improving method according to  claim 6 , wherein a lower limit of the dimensionless distance G is controlled in a case where an inner surface of the tubular body is cooled by water. 
     
     
       13. A tubular-body residual-stress improving apparatus including:
 rotationally driving means which moves circling around an outer circumference of a cylindrical tubular body and whose speed of moving in a circumferential direction can be controlled; 
 an optical head which is held by the rotationally driving means, the optical head locally irradiating an outer-circumferential surface of a welded portion of the tubular body with laser beams, and being capable of adjusting an irradiation area; and 
 regulating means which regulates the rotationally moving means and the optical head, and 
 the apparatus improving residual stress of the tubular body by making the irradiation area of the laser beams move in the circumferential direction, wherein 
 a plurality of thermometers are installed on a tubular body to be improved, and 
 the regulating means:
 makes the plurality of thermometers measure a temperature history of an outer surface of the tubular body while the outer surface is irradiated with the laser beams; 
 obtains a highest achieving temperature, a temperature-rising time required for reaching the highest achieving temperature, and a heating length in an axial direction in which the highest achieving temperature stays within a predetermined temperature range; and 
 controls a heating width in the circumferential direction on the basis of a product of the temperature-rising time and a speed of moving the laser beams in the circumferential direction. 
 
 
     
     
       14. A tubular-body residual-stress improving apparatus including:
 rotationally driving means which moves circling around an outer circumference of a cylindrical tubular body and whose speed of moving in a circumferential direction can be controlled; 
 an optical head which is held by the rotationally driving means, the optical head locally irradiating an outer-circumferential surface of a welded portion of the tubular body with laser beams, and being capable of adjusting an irradiation area; and 
 regulating means which regulates the rotationally moving means and the optical head, and 
 the apparatus improving residual stress of the tubular body by making the irradiation area of the laser beams move in the circumferential direction, wherein 
 preliminarily, a plurality of thermometers are installed on a different tubular body having a same condition as that of a tubular body to be improved, and the regulating means:
 makes the plurality of thermometers measure a temperature history of an outer surface of the different tubular body while the outer surface is irradiated with the laser beams; 
 obtains a highest achieving temperature, a temperature-rising time required for reaching the highest achieving temperature, and a heating length in an axial direction in which the highest achieving temperature stays within a predetermined temperature range; and 
 obtains a heating width in the circumferential direction on the basis of a product of the temperature-rising time and a speed of moving the laser beams in the circumferential direction, and 
 
 subsequently, when the tubular body to be improved is irradiated with the laser beams, the regulating means controls the highest achieving temperature, the temperature-rising time, the heating length in the axial direction, and the heating width in the circumferential direction as treatment conditions. 
 
     
     
       15. The tubular-body residual-stress improving apparatus according to  claim 13  or  14 , wherein
 the regulating means obtains a dimensionless time F=(τ×k)/h 2 , where τ is the temperature-rising time, k is a thermal diffusivity of the tubular body, and h is a wall thickness of the tubular body, and 
 the regulating means controls the dimensionless time F as the temperature rising time. 
 
     
     
       16. The tubular-body residual-stress improving apparatus according to  claim 15 , wherein the regulating means controls the dimensionless time F with an upper limit and a lower limit thereof. 
     
     
       17. The tubular-body residual-stress improving apparatus according to  claim 13  or  14 , wherein
 the regulating means obtains a dimensionless distance in the circumferential direction G=W/√(rh), where W is the heating width in the circumferential direction, r is an average radius of the tubular body, and h is the wall thickness of the tubular body, and 
 the regulating means controls the dimensionless distance G as the heating width in the circumferential direction. 
 
     
     
       18. The tubular-body residual-stress improving apparatus according to  claim 17 , wherein the regulating means controls a lower limit of the dimensionless distance G. 
     
     
       19. The tubular-body residual-stress improving apparatus according to  claim 13  or  14 , wherein
 the regulating means obtains a dimensionless distance in the axial direction J=L/√(rh) where L is the heating length in the axial direction, r is the average radius of the tubular body, and h is the wall thickness of the tubular body, and 
 the regulating means controls the dimensionless distance J as the heating length in the axial direction. 
 
     
     
       20. The tubular-body residual-stress improving apparatus according to  claim 19 , wherein the regulating means controls the dimensionless distance J so that the dimensionless distance J is not less than 3.0. 
     
     
       21. The tubular-body residual-stress improving apparatus according to  claim 13  or  14 , wherein the regulating means controls the highest achieving temperature:
 so that the highest achieving temperature is not lower than 550° C. and lower than 650° C. in a case where the tubular body is made of an austenitic stainless steel; 
 so that the highest achieving temperature is not lower than 550° C. and lower than 650° C. in a case where the tubular body is made of a nickel-chromium iron alloy; and 
 so that the highest achieving temperature is not lower than 500° C. and lower than 595° C. in a case where the tubular body is made of any one of a low-alloy steel and a carbon steel. 
 
     
     
       22. The tubular-body residual-stress improving apparatus according to  claim 13  or  14 , wherein the regulating means controls a product (v×h) so that the product is not less than 70 mm 2 /s, where v is the moving speed of the laser beams and h is the wall thickness of the tubular body. 
     
     
       23. The tubular-body residual-stress improving apparatus according to  claim 15 , wherein the regulating means controls a lower limit of the dimensionless time F in a case where an inner surface of the tubular body is cooled by water. 
     
     
       24. The tubular-body residual-stress improving apparatus according to  claim 17 , wherein the regulating means controls a lower limit of the dimensionless distance G in a case where an inner surface of the tubular body is cooled by water.